This invention relates to a method for removing sulfur generated in nickel processes, such as nickel matte leaching, from the leaching cycle. According to the method, anolyte formed in nickel electrowinning is neutralized using a lime-based neutralizing agent, wherein sulfur is removed from the leaching cycle as gypsum.
It is known before that nickel concentrates are treated pyrometallurgically first in a smelting furnace to obtain nickel matte, which has a relatively high sulfur content. The nickel matte is in turn forwarded to a converter, from where the high grade nickel matte recovered has a considerably lower amount of sulfur. Recently, the pyrometallurgical process portion of the nickel production has begun to be modernized, resulting in the exclusion of the converter altogether, and the nickel matte obtained from the first smelting furnace eg. suspension smelting furnace is routed directly for hydrometallurgical processing. Slag generated in the first smelting furnace usually undergoes further treatment in an electric furnace, from where the matte produced is also directed further for hyrdometallurgical processing. In the first smelting furnace in particular, the sulfur content of the forming nickel matte is always somewhat higher than from a converter and therefore this adds to the amount of sulfur circulating in the leaching process and may cause so-called sulfur swelling, if sulfur removal from the leaching cycle is not taken care of in some way. The problem is typical for a sulfate-based nickel leaching process.
For the purposes of hydrometallurgical processing, the nickel matte is milled and leached to an anolyte of nickel electrowinning. The leaching of nickel matte is a multi-step process and comprises several leaching steps that take place both in atmospheric and pressurized conditions. The resulting nickel sulfate solution is conducted via the solution purification step (cobalt removal) on to either nickel electrowinning or reduction with the aid of hydrogen into metal.
When the nickel sulfate solution formed in the leaching of nickel matte is conducted to electrowinning, the reaction that occurs there is as follows:
NiSO4+H2O== greater than Ni+H2SO4+xc2xdO2xe2x80x83xe2x80x83(1)
The electrodes in the electrowinning process are isolated from each other by a diaphragm. One of the main features of the process is that a certain current has to be conducted through the diaphragm so that the sulfuric acid generated in electrowinning does not come into contact with the cathode, which would result in hydrogen gas and not nickel being formed on the cathode. Thus solution flow for a specific amount of nickel is constant and a certain constant amount of anolyte is produced. The composition of the anolyte is approximately 70g/l Ni and 45-50 g/l H2SO4. As can be seen in reaction (1), the amount of sulfuric acid generated in the electrowinning is molecularly equivalent with the amount of nickel precipitated. At least a part of this oxidized sulfur should be removed from the leaching cycle as sulfate, so that nickel does not exit with the sulfate.
Removal of sulfur from the leaching cycle has been generally carried out by using sodium-based neutralizing agents (sodium carbonate, sodium hydroxide), in other words oxidized sulfur (sulfuric acid) is neutralized with sodium hydroxide or soda depending on the situation. Soluble sodium sulfate is created as a result of the neutralizing process, which under normal circumstances should be removed from the process as a by-product. However sodium sulphate is such a valuable product, that it should be marketable, and if such markets do not exist, the use of sodium-based neutralizing agents in the neutralization of sulfur is not economically viable.
In general, an obstacle to the use of lime in sulfate processes has been the gypsum precipitates which block filter and diaphragm cloths and also cause layers of gypsum nearly to hot surfaces.
A method has now been developed according to the present invention, where at least a part of the sulfur generated in the leaching of nickel matte is recovered by conducting part the anolyte from electrowinning to a secondary flow, where the sulfuric acid is neutralized with some calcium-based compound such as burnt lime (CaO) or limestone (CaCO3). The neutralized solution is saturated with gypsum, and this gypsum is separated from the solution for example with an extractant so that the gypsum content of the solution remains clearly under the saturation level. The neutralized anolyte, which has had some of its gypsum separated out, is channelled back to the leaching cycle, preferably to a cobalt removal step, wherein it does not burden the actual nickel matte solution. The essential novel features of this invention will become apparent from the appended patent claims.
In the method of the present invention, the harmful effects of the gypsum created in lime precipitation can be prevented not only by precipitating and filtering the solution, but also by removing the gypsum from the saturated solution immediately after lime neutralization. Gypsum removal is carried out using a technique known as such, for example by extracting with a suitable extractant such as DEHPA, so that the gypsum content in the solution is clearly below the saturation level, which minimizes the harmful effects of the gypsum. The gypsum precipitate generated can be stored or used as a building material.
FIG. 1 shows a flow chart of the method.